soft parts hollowed. In Sweden there are some large detached masses of granite, containing perforations produced by this cause, some so very large as to admit of a horse and cart passing through them. These effects may be considered as chiefly mechanical. As surely, then, as any part of the earth's crust is elevated into the atmosphere, just as certainly is it liable to be worn down and carried into a lower level.

with it 60 miles from the river's mouth. According to Mr Lyell, the quantity of solid matter brought down by this river every day, is equal in bulk to the greatest of the Egyptian pyramids. According to Captain Sabine, the muddy waters of the Amazon river may be distinguished 300 miles from its mouth.

The constant action of the sea upon the land is strikingly apparent to the inhabitants of coasts. Whole When water collects into channels and follows its islands have been destroyed by the action of tides and well-known tendency to find the lowest level to which oceanic currents, while the remains of others rise above it has access, it becomes a mechanical instrument of the surface of the water, like the ruins of some desostill greater force for wearing down the land. In its lated city. Many instances of the encroachment of smallest rills, as it descends the mountain side, it cuts the sea upon the land have been recorded. An inn on into the soil, and carries off whatever particles it can the coast of Norfolk, built in 1805, then 70 yards from disengage. When gathered into brooks, its operations the sea, was, in 1829, separated from the coast by only are still more powerful. When one of these is placed a small garden. A church on the coast of Kent, which amongst mountains, every heavy shower swells it into in the reign of Henry VIII. was a mile inland, is now an impetuous river, by which large quantities of only about 60 yards from the water's edge. The island detached rock and soil are brought down. In the upper of Nordstrand, on the coast of Schleswig, was, in the parts of the courses of almost all rivers, the greater thirteenth century, 50 miles long and 35 broad. About speed of descent makes up for the smaller volume of the end of the sixteenth century, it was reduced to an water, as far as the power of bringing down stones and area of only 20 miles in circumference. The inhabisoil is concerned. Again, in the lower part of the tants erected lofty dykes for the purpose of saving their course, the smaller speed is sometimes compensated by territories; but in the year 1634 a storm devastated the unevenness of the course; in which case, the water the whole island, by which 1340 human beings and is incessantly driven from one projection of the banks 50,000 head of cattle perished. Three very small islets against another, and by that means wears away a are all that now remain to point out the place where great quantity of solid matter. Many facts have once flourished the fertile and populous island of Nordbeen collected to prove the great efficacy of rivers strand. It is an old notion that Great Britain was once in wearing down the land. The Nerbuddah, a river united to the continent of Europe; and the identity in of India, has scooped out a channel in basaltic rock, structure of the opposite coasts of the strait of Dover 100 feet deep. The river Moselle has worn a channel seems to favour the supposition. There is reason to in solid rock to the depth of 600 feet. Messrs Sedg- believe that the Island of Ceylon was at one period wick and Murchison give an account of gorges scooped united to Hindostan. Humboldt is of opinion that the out in beds of the rock called conglomerate, in the West India islands once constituted a circuit of land valleys of the Eastern Alps, 600 or 700 feet deep. A which enclosed the Gulf of Mexico. stream of lava, which was vomited from Ætna in 1603, happened to flow across the channel of the river Simeto. Since that time the stream has cut a passage through the compact rock to the depth of between 40 and 50 feet, and to the breadth of between 50 and several hundred feet. The cataract of Niagara, in North America, has receded nearly 50 yards during the last forty years. Below the Falls, the river flows in a channel upwards of 150 feet deep, and 160 yards wide, for a distance of seven miles; and this channel has manifestly been produced by the action of the river.

Sometimes, during floods, rivers produce great changes in very short periods. A flood caused by the bursting of the barrier of a lake in the valley of Bagness, Switzerland, moved at first with the tremendous velocity of 33 feet per second. From the barrier burst by the waters to Lake Geneva, there is a fall of 4187 Paris feet; the distance is 45 miles; and the water flowed over all this space in five hours and a half. It carried along houses, bridges, and trees; and masses of rock equal in size to houses, were transported a quarter of a mile down the valley.

The matter carried down by rivers is often deposited at their sides, when it constitutes what is called alluvial land. Sometimes it is deposited at the bottom of lakes, when it forms what are termed lacustrine deposits. In many instances it has been deposited in large quantities at the mouths of rivers, giving rise to what are denominated deltas. Deltas are so called on account of their resembling the fourth letter of the Greek alphabet. The triangular form of a delta is produced by the river, at a certain point inland, dividing itself into two streams which gradually diverge till they reach the ocean, enclosing the space which constitutes the delta. As an instance of the great amount of new land formed at the mouths of rivers, the delta of the Ganges is 220 miles in one direction by 200 in another. The lower part of this delta, a wilderness inhabited by tigers and crocodiles, is as large as the principality of Wales!

The matter carried down by rivers, and thus deposited, is nothing in amount compared to that transported to the ocean. The quantity of sand and mud brought down by the Ganges to the Bay of Bengal, is in the flood season so great, that the sea is discoloured

It thus appears that there are causes in continual operation, for the wearing down of the elevated parts of the earth's crust, and taking the component particles into lower levels. The effects of these causes may be easily traced in the aqueous rocks, many of which are simply deposits of sediment carried by water from high into low places, and subsequently hardened, probably by heat from below and pressure from superincumbent materials. Were such causes not in some way counteracted, dry land could not long exist: all would be taken down and buried in the sea. We find the counteraction in what are termed the Elevating Causes.

Elevating Causes.

As Degrading Causes are chiefly owing to water, Elevating Causes are chiefly owing to fire. They are therefore sometimes comprehended under the term Igneous Agency.

The manifestations of igneous agency at present observable may be considered under three headsnamely, volcanoes, earthquakes, and gradually elevating forces. These phenomena may be viewed as the effects of subterranean heat, operating under different circumstances. A volcano may be described as an opening in the earth's surface, bearing the general appearance of a vent of subterraneous fire, and through which smoke, cinders, and ashes, are almost continually issuing, but which sometimes discharges great fragments of rock, and vast quantities of melted rocky matter. The general effect is a throwing up of earthy material from a low to a high level.

Geographers at present reckon about 200 volcanic vents in activity throughout the earth. The greater number of the whole are in a line along the west coast of South and North America. There are many in the islands of the Pacific and Indian Oceans, and in the centre of Asia. In Europe, there are only three in great activity-Etna in Sicily, Vesuvius in Italy, and Hecla in Iceland. But a vast number of hills throughout France, Britain, and other countries, bear the appearance of having once been active volcanoes. As volcanic action often takes place in the sea, and as there are probably many on land not yet described by geo

The causes of earthquakes have not as yet been In various parts of the world, land is raised out of the satisfactorily explained, but they are now generally sea by the efforts of coral insects. The works of these allowed to be connected with volcanic agency. They creatures are seen upon a vast scale in the Pacific, occur less frequently, and generally with less tremen- where whole ranges of islands are formed by them. dous effect, in Europe, than in some other parts of the On the coast of New Holland, there is a coral reef which world, those parts where volcanic agency is most active, stretches out to a thousand miles in length. The insects being also the parts where earthquakes are most fre- do not commence their laborious operations at a great quent and most dreadful. Though their effect is some-depth below water; from 60 to 100 feet is considered

bottom.

If we consider the operation of the elevating causes, we can be at no loss to understand how we should now see, as composing dry land, and sometimes in very lofty situations, strata which were once at the bottoms of seas; neither will it be surprising, if the irregular nature of volcanic forces is considered, that the strata, so elevated, rarely are found in their originally level position, but in all degrees of inclination, sometimes quite on edge, and even in certain rare instances folded backwards, so as to be upside down.

the utmost extent to which the islands extend down- | stony matter, and had firmly glued themselves to the wards. They are generally of a circular or oval shape; and Mr Lyell is of opinion that corals build upon the rims and in the craters of submarine volcanoes. The outer wall of the building emerges first above the waves, enclosing a pool of tranquil water. The seeds of vegetables are either brought there by sea-birds, or wafted by the ocean, and the islands soon become clothed with a mantle of green. The substance of which these islands and reefs are composed, is lime, which the insects secrete from the sea-water, and cement together with a glutinous matter contained in their bodies. Mr Lyell, while surveying the Isthmus of Panama, detached a quantity of these animals, and placed them on some rocks in a shallow pool of water. On returning to remove them a few days afterwards, he found they had secreted

The changes produced by the united operations of aqueous and igneous agency are in part represented in the subjoined engraving of a supposed section of part of the earth's crust.

a Primary Rock, which has been thrown up, so as to disturb and mix itself with the Secondary Rocks. Secondary Rocks, thrown into inclinations and curves by the rising of the Primary Rock.

c Tertiary Formation, deposited in a hollow formed by the disturbance of the Secondary Rocks.

The circles are Boulders or detached stones, rounded by travelling in water, and deposited in hollows formed by water. The dots indicate beds of gravel, immediately beneath the soil.

PRIMARY.

Granite.

Geologists have been accustomed to describe this as the lowest and oldest of all rocks. Certainly, no other rock is ever found beneath it, except in peculiar circumstances afterwards to be described: if the mass of the earth, therefore, were to be judged of from the small superficial crust with which we are acquainted, granite might appear to constitute the bulk of our planet -a vast nucleus on which all the stratified rocks rested. Geologists are now disposed, under a sense of their limited knowledge, to speak of granite, not as the lowest and oldest of all rocks, but as the lowest as yet discovered, and as one which, though in most of its forms old, is yet sometimes found of recent formation. Granite, in fact, often appears as a volcanic rock, which has been thrown up in a state of fusion through superincumbent strata of all kinds, penetrating into their chinks, and spreading over them on the surface. Even tertiary rocks are found permeated and covered by it -a proof that it has been formed since the deposition of those rocks, which is one of the most recent events in geological chronology. These are the peculiar circumstances in which it may be said that other rocks sometimes lie beneath granite.

Granite, then, may be described as generally forming a basis or bed for all the other rocks as rising in some places from its unmeasured depths into chains of lofty hills and as in other places penetrating in veins through

superincumbent rocks, and partially covering them at the top. It composes some part of the mountain ranges of Cornwall, Cumberland, and the Scottish Highlands; and veins of it are found upon or near the surface in many other parts of England and Scotland.

Three substances usually enter into the composition of granite; namely, (1) quartz, a grey glassy substance, composed of the oxygen of the atmosphere in union with one of the newly discovered metallic bases (silicium); (2) felspar, also a crystalline substance, but usually opaque and coloured pink or yellow, composed of sandy and clayey matter, with a small mixture of lime and potash; (3.) mica, a silvery glittering substance, which divides readily into thin leaves or flakes, and consisting principally of flint and clay, with a little magnesia and oxide of iron. In some granites, instead of mica, we find hornblende, a dark crystalline substance, composed of alumina, silex or flint, and magnesia, with a considerable portion of the black oxide of iron. Such granites are called Syenite, from having been first found in the island of Syene. Other varieties are-Serpentine, in which there are dark spots like those on the skin of the snake (hence the name), and Porphyry, of which the distinguishing peculiarity is its containing little angular pieces of felspar enclosed in the mass.

In man's economy, granite is a rock of great impor. tance. Its uncommon hardness makes it very suitable for the erection of buildings where great durability is desired. Hence, the docks of Liverpool, Waterloo Bridge in London, and many other buildings of a similar nature in England, have been composed of it, notwithstanding that it had to be brought from a great

distance. Nearly the whole city of Aberdeen is built of the granite found in the neighbourhood; and the houses have consequently a glittering appearance when the sun is shining upon them. This stone is also the component material of Memnon's head and Pompey's Pillar, two ancient structures in Egypt.

Inferior Stratified Series.

Above the granite, in its ordinary position, lies the inferior stratified series, consisting mainly of two kinds of rock, gneiss and mica-slate, with alternating strata of hornblende rock, quartz rock, eurite, talcose slates, chlorite slates, and argillaceous slates; of all of which it may be said that they follow no determinate order. These rocks are of the same materials as granite, in a very slightly modified form, and they are nearly as crystalline in their texture. Geologists also find in many places that the granite passes into them-a term expressing a blending of the characters of rocks at the line of their juncture. These two facts have led to the supposition, that the inferior stratified rocks were formed from the materials of the granite, disintegrated by mechanical or chemical means, and washed into the beds of vast oceans, where, on their deposition, they were reached by the high temperature of the interior, and thereby reconsolidated in a crystalline form. To account for the rocks composed exclusively of one of the materials of the granite, we may suppose a chemical separation of those materials.

The most prevalent rock of the series is gneiss, a compound, like granite, of quartz, felspar, mica, and hornblende, and so highly crystalline as to be sometimes scarcely distinguishable from granite. A great portion of the Highlands of Scotland is composed of strata of gneiss, of vast thickness. It is remarkable for its richness in veins of the metals. Mica-slate, or schist, the next most prevalent rock of the series, is composed of mica and quartz. It is the surface rock of many extensive tracts of country. Quartz rock, which we may suppose to have been formed by a che mical separation of that component of granite, is also a prevalent rock. Humboldt takes notice of a mass of it in South America, more than 9500 feet in thickness. The round white pebbles, or candy stones, so often found on sea-beaches, and in the beds of rivers, are pieces of quartz rock. Eurite, of which felspar is the main ingredient, and hornblende rock, the chief element of which is signified by its name, may also be accounted for by a chemical origin.

subservient to the developement of national talents, the highest that have ever been known of their class, for it is the marble from which the works of the Greek and Italian sculptors have been formed. In the geological history of our globe, its first appearance in the ascending series of rocks is an event of no small consequence, for limestone strata form a large proportion of the superior formations, and the manner in which they have been formed has engaged much attention. Limestone is the carbonate of lime, that is, a combination of the earth lime (itself an union of the metal calcium and oxygen) with carbonic acid (this being, again, an union of oxygen with the elementary substance carbon). Carbon is the largest element in the composition of vegetable and animal substances, and this its first appearance in the structure of rocks is of course a point of much interest, more especially as it is generally concluded that many of the superior limestone strata have been entirely formed of animal remains. We are thus tempted to surmise that the formation of the limestone beds of the inferior stratified series marks some early and obscure stage of organic existence on the surface of our planet. No distinct remains of plants or animals have, indeed, been found in this series; and it is customary to point to the next upper series, in which both do occur, as the era of organic life. Yet many geologists are of opinion that the inferior stratified rocks might have contained such remains, though the heat under which the rocks seem to have been formed may have obliterated all trace of such substances.

TRANSITION.

Grauwacke Group.

All the rocks hitherto described are of crystalline texture, and, apparently, chemical phenomena have arrived at, traces of mechanical origin and deposition attended their formation. In the group we have now become apparent; but still a few strata resembling the preceding occur throughout the lower parts of this series, as if the circumstances under which the earlier rocks were formed had not entirely ceased. Hence the term transition for the series, as implying a passing from one state of things to another.

The rocks forming the lower part of this group, and which are sometimes separately classed as the Lowest Fossiliferous Group, are an alternation of beds of chlorite, talcose, and other slates, resembling those of the inferior stratified series, with beds of clayey and Clay-slate is the geological term for the well-known sandy slate, of apparently mechanical origin, and in which a few fossils are found. It thus appears that the stone with which houses are roofed. It is, as its name cessation of the chemical origin of rocks, and the comimports, composed mainly of clay-a substance too mencement of organic life, are events nearly connected; liberally diffused amongst the ingredients of granite, to and it has thence been surmised that the temperature of admit of any wonder as to its being found in a nearly the earth's surface was now for the first time suitable to distinct state in this rock. Mica-slate and clay-slate the production and maintenance of organic things. At are fissile in their structure—that is, capable of being the same time, the alternation of the rocks teaches us split into very thin plates: hence the utility of slate, the instructive fact that the change was not direct or as a material for covering houses. But a curious diversity exists in this respect between mica-slate and roof- uniform, but that, for some time, the two conditions of ing slate. In the former, the cleavage, or direction in with a general observation, which has been made by the surface superseded each other. This is conformable which it splits, is in the same line as the stratification; an eminent geologist,* namely, that, however sudden but in roofing slate, the cleavage is always more or less changes may have taken place in particular situations, transverse. What makes the latter circumstance the more remarkable-when strata of roofing slate are mations is usually seen to have been more or less a general change of circumstances attending rock forfound, as often happens, contorted or wavy, the direction of the cleavage is in one straight line through them all, gradual. The few fossils found in this part of the series indicating that the influence which produced the cleavare, as far as ascertained, the same as those of the next age in that rock took effect after the whole had been laid higher rocks. down, and after, by some subsequent accident of pressure, they had been forced into a wavy direction. Probably this phenomenon is of an electric nature Clay-slates are found in great abundance in Cornwall and in the Scottish Highlands. A fine kind makes the slates used at school, and from a kind still finer are cut the pens used for writing on school-slates.

In the inferior stratified series, there occur a few small beds of limestone, sometimes called Saccharine Limestone, from its resemblance to refined sugar, and sometimes Primitive Limestone, from the period of its occurrence in the series. In Greece and Italy this rock has been

These are a series of arenaceous and slatey rocks, of evidently mechanical origin, intermixed with small beds of limestone, in which that peculiarity is less distinct, the whole being termed more particularly the Grauwacke indicates its having been formed of a fine detritus group. The general composition of the grauwacke (matter washed from other rocks), and its having been rock, of various sizes under that of a man's head, deposited slowly; but it sometimes has fragments of imbedded in it, and it occasionally passes into conglomerates. The limestones mixed with the grauwacke *M. de la Boche-Manual, 474.